The volumetric and gravimetric utilization of active materials in current lithium rechargeable batteries is much less than 50%, due to several factors. Particulate-based electrodes currently used in lithium rechargeable batteries are prepared by blending of a starting powder with polymer binder, solvent, and conductive additive powders such as carbon. The blended mixture is then coated onto metal foil current collectors, dried and calendared under high pressure to obtain a densified coating. These electrodes have poor packing density of the active material, typically less than 60% by volume, and for adequate rate capability the open porosity is typically 30%-40% by volume. A substantial fraction of the electrode volume is also occupied by polymer binders that are introduced to improve processing of the electrodes, and carbon additives that are added to improve electronic conductivity of the electrode. Carbon additives are especially necessary in electrodes that use poorly electronically conducting compounds such as intercalation oxides and polyanion compounds. This is because the powder particles of such compounds, when compacted, tend to form highly resistive “point” contacts between the hard non-deforming particles, resulting in a poorly conducting network. These electrode additives have low specific gravity and thus occupy substantial volume.
Furthermore, transport of lithium ions and electrons in such electrodes is poor and requires relatively thin electrodes, usually less than 150 micrometers thick, in order to achieve complete utilization of the active material. The thin laminate structure of such rechargeable batteries necessitates large fractions of non-storage materials including binders, conductive additives, current collector foils, separator films, and electrolyte, leading to the aforementioned poor volumetric and gravimetric utilization. In high power batteries, the thermal conductivity of electrodes should also be high to prevent excessive heating during cycling that causes degradation of energy and power and limits the life of the battery. Particulate-based electrodes have poor thermal conductivity since heat must be transported from particle to particle through narrow point contacts. These limitations are further exacerbated in positive or negative electrodes that use intercalation oxides or polyanion compounds such as phosphates or sulfates, since such compounds have poor thermal conductivity.
Another aspect limiting the utilization of traditional battery structures is the requirement that each electrode is connected to a common terminal. In a wound cell, the point of connection is through a tab extending from the side of each electrode. The volume and surface area of this tab limits the amount of current that can flow through the tab and be accessible to the device.
Thus it is of value to have an electrode that has a high packing density of the active material, fast electron and ion transport kinetics, high thermal conductivity, and that can be prepared as a thick electrode while still having high electrochemical utilization.
International Patent Publication No. WO 08/153749, filed May 23, 2008 to Chiang et al., entitled “Batteries and Electrodes for Use Thereof” which is incorporated by reference in its entirety generally relates to batteries or other electrochemical devices, and systems and materials for use in these, including novel electrode materials and designs. The publication further discloses batteries having high power and minimal volume.